The use of piezoelectric actuators in fuel injection systems is well documented. FIG. 1 is a sectional view of a known fuel injector 1 for an internal combustion engine. The injector 1 includes a piezoelectric actuator 2 having a piezoelectric stack 4 which is housed in a sheath or encapsulation 6, which is typically polymeric or metallic, to protect the actuator 2 from its environment. The encased actuator 2 is located within a fuel volume, referred to as the stack volume 8, which in use is filled with fuel at injection pressures. The actuator 2 is coupled to a valve needle 10 so as to control movement of the needle 10 towards and away from its seating 12b, thus controlling the supply of fuel through fuel channels 12a in the nozzle 12 to a cylinder of a combustion engine (not shown).
The piezoelectric stack 4 comprises a multilayer structure 14, a section of which is shown in FIG. 2. The multilayer structure 14 includes a plurality of relatively thin ferroelectric ceramic layers 16 separated by a plurality of internal electrodes forming first and second electrode groups 18a, 18b. An example of a ferroelectric material used in piezoelectric actuators is lead zirconate titanate, commonly referred to as PZT (Pb(ZrxTi1−x)O3).
The electrodes of the first group 18a are interdigitated with the electrodes of the second group 18b. The electrodes of the first group 18a connect with a first external electrode 20a and the electrodes of the second group 18b connect with a second external electrode 20b. In use, the first and second external electrodes 20a, 20b receive an applied voltage that produces an intermittent electric field between adjacent interdigitated electrodes 18a, 18b, which causes the stack 4 to extend and contract along the direction of the applied field. As a result, the valve needle 10 is operated so as to control injection into the associated engine cylinder.